1. Field of the Invention
The present invention relates to USB devices which perform communications through a USB interface.
2. Background Art
Recently, the interface as specified by the USB standard has widely been used. FIGS. 3A and 3B are explanatory diagrams for roughly explaining an electrical connection when devices are connected by use of a USB interface. In the figure, reference numeral 31 is a host-side device; 32 is a host-side interface chip; 33 to 34 are resistors; 35 is a cable; 36 is a device-side device; and a 37 is a device-side interface chip. The USB is a serial interface of four core wires, and the host-side device 31 is connected to the device-side device 36 by a four core-wire cable 35. The four core wires are a power-source supplying line (Vbus), a ground line (CND), and two data lines (D+ and D−).
The data lines are connected to the host-side interface chip 32 in the host-side device-31 and the device-side interface chip 37 in the device-side device 36, and used for transferring data between both the chips. The two data lines are earthed through resistors 33 and 34, respectively. The resistors 33 and 34 maybe resistors of about 15 kΩ. In the device-side device 36, when it is a full speed device, the data line D+ is connected to the power source via the resistor 38 as shown in FIG. 3A. When it is a low speed device, the data line D− is connected to the power source via the resistor 38 as shown in FIG. 3B. The resistor 38 has a resistance of about 1.5 kΩ.
In the USB standard, electric power may be supplied from the host-side device 31 to the device-side device 36 by use of the power-source supplying line and the ground line. A maximum power specified in the USB standard is defined by 5V and 500 mA. Accordingly, for the connection of a device of relatively low power consumption, e.g., a computer mouse, as the device-side device, no external power source is required. Then, enhanced convenience is secured, and provision of low cost device is realized.
The devices that are connected by utilizing the USB interface are not only devices that consume low power. Those devices include devices consuming larger electric power and devices requiring voltage other than 5V. Also the device of the assembling type is constructed to utilize the USB interface for the connection of related parts thereof. Since those related parts are customized, the power source specifications are frequently different from the standard specifications of the USB interface. The power sources of 3.3V and 24V, for example, are frequently needed in addition to the power source of 5V.
In a case where the standard electric power prescribed by the USB interface standards is insufficient for the electric power consumed by the device-side device or the power voltage of the USB interface is not coincident with the required power voltage, another power source is separately provided. If another power source is installed to the device-side device in addition to the power source for the host-side device, the cost of the device-side device is much increased. It is a common practice that the power source is additionally provided, in the device as separately sold, such as printers or scanners, however. Installation of some power sources in one device, as in an assembling type device, leads to increase of the device, and is unacceptable since those will excessively occupy the limited inner space of the device.
For the difference of only the power voltages, a measure is easy; for example, a DC-DC converter is provided in the device-side device and converts the voltage of 5V to a voltage of a desired value. The measure is incapable of supplying sufficient electric power to such a device that needs electric power which exceeds the standard electric power of 5V and 500 mA.
Another possible solution to this power source problem is present. In the solution, power source wires are provided between the host-side device and the device-side device, and necessary electric power is supplied to the device-side device through the wires, in addition to the power source of the USB. This approach is equivalent to the case using the dedicated interface, and advantages brought about by using the standardized interface, or USB, is much reduced.
Thus, the USB interface alone is incapable of supplying electric power to the device-side device which consumes electric power in excess of the standard USB power supplying ability and the device-side device customized to have specific power source specifications.